It was while diving the shipwrecks of the St. Lawrence near Kingston and Brockville, Ont., and in Lake Huron off Tobermory, Ont., that Jill Heinerth got her first taste of the mysteries that lie at the bottom of Earth’s lakes and oceans. While exploring an underwater grotto off Tobermory called “The Caves” for the first time, Heinerth drew a deep breath, and as she rose through the stunning, cathedral-like space, she was hooked.

Soon, evening and weekend dives weren’t enough, and by 1991 Heinerth had traded her career in graphic design for one in cave diving. Since then, she has explored the planet’s watery depths from Antarctica to Russia. “When I swim through a cave, I feel as though I’m swimming through the veins of Mother Earth,” says the recipient of the inaugural Sir Christopher Ondaatje Medal for Exploration.

In 1998, Heinerth joined the team that made the first 3D map of a cave system, in Wakulla Springs, Florida. During one mapping mission, more than 90 metres below the surface, Heinerth and her exploration partner Brian Kakuk finished their tasks early. They decided to continue to explore, breaking into a virgin cave. Afterwards, Heinerth learned that she had swum more than 3,000 metres — farther into a deep cave than any woman, ever. But more important, she says, was the success of the new instrument that she and the rest of the team used: NASA now plans to use such a device to map the oceans of Jupiter’s moon Europa.

While swimming through underwater caves, Heinerth has witnessed “humankind’s ignorance” of the ways in which our actions affect our water resources. She has seen the history of climate change in sea-level notches deep beneath the surface of the ocean. That inspired her to make a difference. Heinerth’s We Are Water project aims to improve “water literacy” through free educational resources. Next, Heinerth intends to snorkel the Northwest Passage with a women’s relay team, to highlight the issue of melting sea ice in the North. “I am swimming in the lifeblood of our planet. I am swimming in your drinking water,” she says. “We all need to know how we can protect it for future generations.”